Shut-off devices of the above mentioned general type are used to control the flow of a conveying fluid, such as air, another gas, or a liquid, and a loose bulk material being conveyed by the conveying fluid in a hoseline or a pipeline or some other type of conduit, for example in a pneumatic conveyor system. In other words, such shut-off devices serve to open and close the conveyor conduit in a controlled manner. Generally, such shut-off devices are used in pneumatic conveyor conduits that convey plastic granulates and the like to various processing machines, whereby for example, a respective shut-off device is opened in order to fill a supply hopper or the like with the loose bulk plastic granulate, and is then closed once the required amount of material has been conveyed.
The conveyed material is generally not absolutely homogeneous in terms of particle size and the nature or quality thereof. Such inhomogeneity applies especially, but not exclusively, to recycled materials. The particulate nature of the conveyed materials, and the inherent inhomogeneity of the particle size and shape and other parameters of the material, lead to problems in the operation of the shut-off devices. For example, when the shut-off device is to be closed, not only the conveying air-stream, but also the flow of conveyed material itself must be blocked or shut off. As a result of the bulky materials becoming caught or hung-up as the shut-off element of the shut-off device is closing, it often becomes necessary to apply a considerable closing force to the shut-off element, especially when particles of the conveyed material must be pinched off by the shut-off element.